One of the problems related to industrial application of micro plasma (micro arc, plasma-electrolyte) oxidation method is its significant energy consumption. At present there are no power supplies that would allow treating large-sized parts or simultaneously processing a large number of parts.
Attempts have been made to reduce energy consumption of the process or apply coating to large-sized parts. Some of those attempts were aimed at selecting electric power supply modes to minimize energy expenditure; others involved mechanical displacement of parts, such as, for instance, motion of parts in relation to each other, counter electrode movement in relation to the processed part or gradual immersion into electrolyte, i.e. stage-by-stage part treatment.
There is a method (RU 2218454 C2, 2003) for obtaining wear-resistant coatings, where a technological electrical insulating layer of inorganic compounds is formed on the base surface before micro arc oxidation. Such layer allows for electric energy saving by ensuring lower energy input into formation of an outer porous technological layer and by reducing starter currents.
The disadvantage of this method is a need to apply electric insulating inorganic barrier, which results in abrupt processability and productivity drop and increases the costs of obtaining a coating. Inorganic insulating barrier is to be uniform all over the part, which is technologically difficult to achieve, and this barrier is relatively hard to apply to irregular shaped parts. Therefore, impossibility of ensuring uniform electric insulating barrier on irregular shaped parts does not allow obtaining high-quality homogeneous coatings by micro arc method, because irregular electric density results in nonuniform coating thickness.
There is a method (RU 2006531 C1, 1994) of electrolyte micro arc application of silicate coating to aluminum parts, which consists in immersing 5-10% of surface area of said part into electrolyte, while further immersion is carried out evenly at a certain speed, depending on initial current density and total surface area of the part. Initial current magnitude is 1000 A, which allows applying a 10-20 times less potent supply source.
Improvement of the above-mentioned method is a method, stipulated in (RU 2065895 C1, 1996), where stage-by-stage immersion of the part is carried out.
There is a method (RU 2149929 C1, 2000; U.S. Pat. No. 6,238,540 B1, 2001), aimed at obtaining high-quality coating for extended surface of a processed large-sized part or for a large number of small parts simultaneously. It is done by facilitating the generation of micro plasma discharges and ensuring their stable combustion. Immersion is stage-by-stage in this process. First, the area is determined depending on the power output; then further immersion, till full submersion of the part, is carried out keeping current magnitude between the electrodes within certain limits.
Gradual immersion of the part into electrolyte causes stage-by-stage expansion of an active micro-arc discharge zone, which can result in heterogeneous distribution of energy input into bare surface depending on time and, correspondingly, in heterogeneous coating properties, i.e. in obtaining low-quality coating. Parts, which were initially placed into solution, will have larger thickness. The whole article passes through electrolyte-air interface, which also causes coating defects. When parts are irregular shaped, it is impossible to ensure constant current density, as it is unpredictable in this case.
There is a method of obtaining protective coatings on the surface of metals and alloys (RU 2194804 C2, 2000), where operational device is moved along the processed surface, and device is equipped with electrode and porous screen, through which liquid electrolyte is brought in. The authors underline that unlike existing oxidation methods, where power supplies enduring current of up to 500 A are used to maintain requested current density, the suggested method is based on the use of 2 kW device, ensuring necessary process parameters to apply coating to large-sized parts.
Disadvantage of this method is a need to use a manipulator, which is to move along the surface of the part. This is especially problematic, when coating is applied to irregular shaped parts, containing holes, cavities etc. Despite the theoretic possibility of applying coatings to large surfaces, this method, however, does it at the expense of increasing the required time. Besides, a crucial disadvantage of applying small cathodes is the fact that when voltage is applied, cathode is polarized to a larger extent than the processed part. As a result significant energy loss at cathode takes place and efficient electric energy use is decreased.
There is a method for electrolytic micro arc coating application to parts made of valve metal (RU 2171865 C1, 2000), designed to obtain coatings on large-sized parts when using low-power supplies. In this method the electrode is given a specific form and an area much smaller than the area of a processed part. Coating application is carried out by electrode scanning along the surface of the part or simultaneous motion of electrode and processed part in relation to each other.
Disadvantage of this method is a need for additional equipment (manipulator), and it is impossible to process irregular shaped parts. From electrochemical point of view, economical processes are viable, when area of processed part is smaller than cathode area. In this case cathode is weakly polarized. If cathode surface is smaller than the surface of processed part, then the main voltage drop takes place on cathode and anode is weakly polarized. Speed of coating formation in this case is reduced and the time requirement increases, as it is necessary to apply coating of a given thickness on one part segment and then move cathode to a different segment. This worsens processability and productivity of this method.